1. Field of the invention
The invention relates to a nuclear reactor with improved efficiency capable of better utilization of the fuel material of the core fuel assemblies.
Nuclear reactors cooled and moderated by pressurized light water comprise a vessel containing the reactor core immersed in pressurized water filling the vessel. The core of the reactor comprises fuel assemblies of great length as compared with their cross-section arranged vertically and side by side. The fuel assemblies themselves consist of bundles of fissile fuel rods in contact by their external surface with the cooling water of the reactor.
For the operation of the reactor, a plurality of control bars associated with certain fuel assemblies of the core are used. Each control bar may consist of a cluster of parallel rods of strongly neutron-absorbing material which can be moved vertically within guide tubes replacing some fuel rods in the fuel assemblies forming the core.
The invention more particularly relates to nuclear reactors moderated and cooled by light water and to a method of operating such a reactor with a sequence which comprises, during a same fuel cycle and before an operating phase with an energy spectrum of the neutrons which is thermal or epithermal, a first phase with a "hardened" neutron energy spectrum as long as the reactivity of the nuclear fuel is high.
2. Prior art
One of the major problems involved in the operation of nuclear reactors is to obtain high efficiency in the use of the nuclear fuel of the fuel assemblies. The fuel generally consists of uranium in the form of uranium oxide predominantly containing uranium 238 which is a fertile material and containing a quantity of fissile uranium 235 which varies as a function of the degree of enrichment of the fuel.
During reactor operation, the fissile material is burnt up so that it is necessary to replace at least part of the fuel assemblies of the reactor after a certain period of operation.
The cost of the operations to enrich, remove, replace the used fuel and reprocess it is very high, so that it is desirable to make the best possible use of the fuel introduced into the reactor core in order to improve the economic operating conditions of the reactor.
It is important to achieve the most complete possible burn-up of the uranium contained in the fuel assemblies. By improving uranium consumption, it is possible either to extend the useful life of the core for a predetermined initial content of fissile or to reduce the initial fissile uranium content uranium in the core for a predetermined useful life. In the former case, the operating costs of the nuclear reactor are reduced by reloading at longer intervals of time. In the latter case, it will be possible, for example, either to reduce the volume and the total mass of the fuel rods of the core, or alternately to use fuel with a lower degree of enrichment. In this way the cost of the fuel charge will be reduced.
In order to operate the reactor, that is to say in order to control the reactivity of the core, neutron-absorbing materials are used either in the form of control rods which are inserted into the core of the reactor, or in the form of elements dissolved in the cooling and moderating water of the reactor. Immediately after a fresh core is loaded, its reactivity is high and it is necessary to use absorbing material in increased quantity for operating the reactor. For example, clusters of rods containing burnable poisons are introduced into the guide tubes of some fuel assemblies of the core, or neutron-absorbing poisons are introduced in considerable quantity into the cooling water.
When the excess reactivity decreases due to exhaustion of the fuel, the concentration of the neutron-absorbing poisons which are dissolved is decreased correspondingly. The neutron-absorbing poisons, which are necessary for the operation of the reactor in its initial state, are expensive and they reduce the yield of the fissile material contained in the core.
It has been proposed to use the excess reactivity of the core in its initial state to produce fissile plutonium from uranium 238 contained in the fuel of the arrays. To do this, the neutron energy spectrum in the core is shifted towards higher energies, by reducing the ratio of the volume of moderator to the volume of fuel in the core, during a first part of the fuel cycle. When the excess reactivity of the fuel becomes close to zero, the moderator/fuel ratio is modified to a value restoring the neutron spectrum to the "thermal" or "slow" range generally used in PWRs. This has the effect of producing a fresh excess of reactivity, which permits to extend the period of use of the fuel.
A first approach for improving fuel utilization in nuclear reactors by shifting the neutron energy spectrum from intermediate to thermal as the fissile material progressively burns up consisted of operating with a mixture of heavy and light water during a first phase of operation.
In an attempt to overcome the drawbacks associated with mixing heavy water and light water, it has been suggested U.S. Pat. No. 4,255,236 (Robbins) to initially run a reactor in an undermoderated condition for increasing the conversion ratio and then to shift the neutron spectrum to lower energies by initially operating the reactor in a boiling water mode and then converting a number of fuel assemblies to a non-boiling mode and/or removing some of the fuel rods from fuel assemblies for increasing the moderating ratio.
All fuel rods used by Robbins are identical and removing some of the fuel rods implies shutting down the reactor, opening the pressure vessel and dismantling some at least of the fuel assemblies.
Another approach consists in modifying the moderator volume/fuel volume ratio by introducing, during the first part of the fuel cycle, rods of neutron-transparent material within some of the guide tubes of the core fuel assemblies. In this way, the water contained in these guide tubes is expelled and the volume of moderator in the core is reduced by this amount.
To obtain an appreciable effect, it is necessary to displace approximately 20% of the cooling water during almost 60% of the useful life of the core. To do this, a very large number of neutron-transparent rods are introduced into all the guide tubes of the core fuel assemblies, with the exception of those used for guiding absorbing rods for control of the reactor.
The conception and design of the reactor must be considerably reviewed and complicated since the core equipments of the reactor must be arranged for guiding the water displacer spectrum variation rods above the core and for moving them. That approach therefore requires a large number of additional guide tubes in a zone of the internal equipments through which hot water flows with a detrimental increase of the head loss in the reactor: it is therefore necessary to redesign the coolant flow paths in the reactor vessel. Moreover, linear actuation of the rod clusters requires a large number of control mechanism carried by the cover of the vessel, in addition to the existing mechanisms for actuating the clusters of control rods for fine control of the reactor. For the same thermal power, there is an increase in the size of the reactor vessel as compared to a conventional reactor.
French patent 2 535 509 describes a method of operating a nuclear reactor moderated and cooled by light water including a core formed by fuel assemblies immersed in light water which are at least partially replaced at time intervals (corresponding to operating cycles of the reactor). Bars of a material absorbing low energy neutrons are introduced in the core for varying the energy spectrum of the neutrons during a first phase of the cycle for reducing the ratio between the moderator volume and the fissile material volume in the core and for shifting the neutron energy spectrum towards higher energies. The spectral shift bars are removed during a second phase of the cycle.
The material of the spectrum variation bars contains fertile nuclei which can be transformed into fissile nuclei under the effect of neutrons, that material may be U-235 depleted uranium oxide.
Capture of the neutrons by the fertile uranium 238 nuclei gives rise to plutonium which may be extracted by reprocessing the spectral shift bars. Since the bars only stay in the core of the reactor during the first phase of the cycle and are then removed into the upper internals of the reactor, their average irradiation at the time of unloading, following an operating cycle, is much smaller than that of the fuel assemblies. Consequently, the plutonium contained in the bars is of much better quality than that obtained from the fuel. This is represented by a higher amount of Pu 239 and a lower amount of isotopes called "chain end isotopes", i.e. Pu 241 and Pu 242. The operating method disclosed in French patent 2 535 509 may consequently cause proliferation, since the construction of a chemical extraction plant raises much less technical and economic problems than that of an isotopic separation plant.
The use of thorium as fertile material, instead of depleted uranium, raises substantially the same problems since, upon neutron absorption, thorium 232 produces uranium 233 which is also fissile and may be separated from the thorium by chemical extraction.